The present invention relates generally to the configuration and deployment of pressure control equipment used in drilling subsea wells. More particularly, the present invention relates to subsea blowout preventer stack systems.
As drilling rigs venture into ever increasing water depths and encounter new challenges, well control has become increasingly problematic. As costs of floating mobile offshore drilling units escalate, traditional time-intensive operations are constantly being re-evaluated in an effort to reduce overall non-drilling time, thereby increasing the drilling efficiency of the rig.
One of the most time-intensive operations is running the riser, which provides a plurality of parallel fluid conduits between the drilling rig at the surface and the blowout preventer (BOP) stack coupled to the wellhead at the seafloor. In order to facilitate handling of the riser on the rig, the riser is usually constructed by connecting a number of joints that are generally less than fifty feet in length. The riser is “run” by connecting a joint of riser to the BOP stack, lowering the riser-connected BOP stack a short distance, and then connecting another joint of riser to the uppermost end of the riser string. This process continues until the BOP stack is lowered to the wellhead at the seafloor.
In water depths in excess of 5,000 ft., running the riser generally takes several days to complete. Thus, minimizing the number of times the riser must be run is critical to minimizing the time needed to drill and complete a well. Since the BOP stack is installed at the very bottom of the riser, attempts to increase the amount of time that the BOP stack can stay on the wellhead are being explored. One factor limiting the time a BOP stack can stay on the wellhead is for maintenance of the ram BOP packer seals. Ram BOP packer seals have a limited useful life and once that limit is reached the ram BOP cannot be used until the seals have been replaced.
One common way to improve the time a BOP stack can stay on the wellhead is to increase the number of useable ram BOP cavities in the BOP stack to the point of having a “primary” and “secondary” ram BOP cavity for each size installed. In this way, the time that a BOP stack can remain operational on the wellhead would be effectively doubled. However, simply increasing the number of ram BOP cavities in a subsea BOP stack presents its own set of new challenges, such as increasing the size and weight of the BOP stack.
Drilling in deep water has often utilized subsea BOP stacks having four to six ram BOP cavities. Increasing the number of ram BOP cavities, such as to eight or ten cavities would increase the weight of the BOP stack, in some cases to a million pounds or more. Many existing rigs do not have the capacity to handle and operate such a BOP stack. In order to safely operate such a system, enhancements would be required to not only the BOP stack handling equipment on the rig, but also to the drill floor equipment, the drawworks and other hoisting equipment, the rotary table, the derrick, and the riser. Enhancing all of this equipment would likely require expanding the basic rig design to allow it to carry the additional weight of all the enhanced equipment systems and provide room for handling and storing the BOP stack.
Thus, there remains a need to develop methods and apparatus for allowing improved redundancy and operational times of subsea BOP stacks in order to overcome some of the foregoing difficulties while providing more advantageous overall results.